Calculate Molarity and Concentration

Question

1. Calculate the molarity of the following solutions:

a. A sample of solid KBr weighing 8.90 g is put in enough distilled water to give 200.0 mL of

solution.

b. A 24.9 g sample of NaCl is dissolved in a quantity of water, and the solution is stirred well. A

500.0 mL of that solution was prepared.

2. Calculate the concentration in %(w/w) of the following solutions. Assume water has a density of 1.00

g/mL.

a. 0.200 mol HCl is dissolved in 150.0 mL of water.

b. 5.20 g of CaCl2 is dissolved in 250 mL of water.

Answer 1

Molarity M = moles of solute / Liters of solution. Watch units!

1a. (8.90g KBr) * (1 mole KBr / 119.0g KBr) = 0.0748 mole KBr; 200.0 mL = 0.2000 L.

M = (0.0748 mole) / (0.2000 L) = 0.374 mole/L.

1b. (24.9g NaCl) * (1 mole NaCl / 58.44g NaCl) = 0.426 mole NaCl.

M = (0.426 mole) / (0.5000 L) = 0.852 mole/L.

I assume "%(w/w)" is percent ratio of solute mass to total mass. As given, each mL of H₂O = 1 g H₂O.

2a. (0.200 mole HCl) * (36.46g HCl / 1 mole HCl) = 7.29 g HCl.

% = 100 * (7.29g) / (150.0g + 7.29g) = 4.64%.

2b. % = 100 * (5.20g) / (250g + 5.20g) = 2.04%.